1. Field of the Invention
The invention relates to a magneto-optical high-voltage current measuring transducer and more particularly to a measuring transducer utilizing light polarization changes created by a magnetic field associated with the high-voltage current.
2. Description of the Prior Art
Magneto-optical measuring transducers are known. In a known measuring transducer, polarized light flows through a first Faraday rotator which is arranged as a measuring sensing element in a magnetic field which is dependent upon the high-voltage current to be measured. On the passage through this Faraday rotator, the polarization direction of a polarized light beam is rotated in dependence upon the magnetic field. The polarized light which emerges from the Faraday rotator and is altered in respect of its polarization direction, now passes through another Faraday rotator, the so-called compensator, which carries earth potential. The compensator is connected to a regulatable magnetic field so that the polarized light, which has been altered in respect of its polarization direction is returned to the original polarization direction. The strength of the regulatable magnetic field is then consequently a gauge for the current strength of the current to be measured.
It is already known to design a Faraday rotator as a light conducting coil. A light conducting coil of this type consists of a glass fibre which serves to convey the polarized light beam, whereby the polarization direction of this light beam is rotated on its path through the glass fibre in dependence upon the prevailing magnetic field.
Such Faraday rotators in the form of light conductor coils have limited measuring accuracy, however. As a result of the curvature of the light conductor fibres in the coil, unsymmetries arise which cause birefringence.
It is known how this curvature-dependent birefringence can be compensated. The starting point is the recognition that in terms of their influence on polarized light, coils composed of light conducting fibres can be described by way of a model as a birefringent crystal. For reasons of symmetry, a main axial direction is identical with the coil axis. In a birefringent crystal, main axial directions are to be understood as those directions of polarization in which a linearly polarized light beam can pass through the crystal without the polarization of the light beam becoming altered.
Each birefringent crystal possesses two main axial directions which are at right angles to one another. If it is traversed by a linearly polarized light beam exhibiting a polarization direction which differs from the main axial direction, elliptically polarized light is formed. If two similar crystals are optically connected in series to one another in such manner that the direction of the main axis with the more rapid light propagation in the one crystal is identical to the direction of the main axis with the slower light propagation in the other crystal, i.e. if the similar main axial directions are crossed, the differences in transit time for different polarization directions are compensated, so that a linearly polarized light beam which enters this crystal combination emerges linearly polarized again, and in fact independently of its polarization direction.
On the basis of this knowledge, it has been proposed that two identical light conductor coils be optically connected in series, the coil axes to be aligned at right angles to one another. The one light conductor coil serves as a measuring sensing element and carries a high voltage potential, whereas the other light conductor coil serves as a compensator and carries earth potential. In this arrangement the curvature-dependent birefringence can be compensated.
However, the strength of the curvature-dependent birefringence is also temperature-dependent and as the measuring sensing element and the compensator are generally relatively widely spaced from one another in order to achieve insulation from the high voltage current to be measured, it is difficult to ensure an equal temperature on the measuring sensing element and on the compensator.